In recent years, technological innovations such as higher densification of semiconductor packages and development of integrated circuits with a larger scale of integration than LSI and high-speed integrated circuits are made in the fields of electronic instruments and dissipation of heat thereby generated presents a theme of extreme importance. One of the measures currently under investigation for dissipating heat is application of heat-conductive articles molded from heat-dissipating materials such as metals, ceramics, and polymer compositions to the heat-dissipating parts such as printed circuit wiring boards, semiconductor packages, box-shaped housings, heat sinks, and heat dissipation sheets.
Of the aforementioned heat-dissipating materials, heat-conductive cured epoxy resins have excellent properties in respect to electrical insulation, mechanical properties, heat resistance, chemical resistance, adhesiveness, and density and are used widely in the electrical and electronic fields as cast articles, laminated sheets, encapsulants, adhesives, and the like.
Epoxy resin compositions containing fillers of high thermal conductivity are known to yield heat-conductive cured products, but none of the cured products obtained thus far from the conventional epoxy resins shows sufficient thermal conductivity.
Some of epoxy resin compositions known for excellent thermal conductivity contain epoxy resins having a mesogenic structure. For example, JP7-90052 A describes an epoxy resin composition containing a biphenol type epoxy resin and a curing agent based on a polyhydric phenol resin as essential components and the document discloses that the composition shows good stability and high strength at high temperatures and is useful for a wide variety of applications such as adhesion, casting, encapsulation, molding, and lamination. Further, JP9-118673 A discloses an epoxy compound having two mesogenic structures linked by a bent chain in the molecule. Still further, JP11-323162 A discloses a resin composition which contains an epoxy resin having a mesogenic group.    Patent document 1: JP7-90052 A    Patent document 2: JP9-118673 A    Patent document 3: JP11-323162 A    Patent document 4: JP2004-123847 A    Patent document 5: JP2005-325231 A    Patent document 6: JP2005-306955 A
The cured products obtained from the aforementioned epoxy compounds having a mesogenic structure are optically anisotropic and some of them have been confirmed to manifest the properties of liquid crystals with a domain size of from several μm to several tens of μm, but none has not gone so far as to form a crystal phase with a domain size of several hundreds of μm. In consequence, the cured products obtained from conventional epoxy resins having a mesogenic structure are anisotropic on a micro scale, but they are isotropic on a macro scale; hence, they are optically transparent when visually observed. Moreover, a distinct endothermic peak attributable to melting of the crystal phase is not observed in differential thermal analysis. That is to say, the cured products obtained from the epoxy resins having a mesogenic structure reported thus far show insufficient growth of the crystal phase and they fail to be sufficiently effective for providing high thermal conductivity, low thermal expansion, high heat resistance, low moisture absorption, and good gas barrier properties. Furthermore, because of insufficient growth of crystals in the said cured products, their heat resistance depends on glass transition temperature and high heat resistance attributable to melting of the crystal phase cannot be expected.
Resin compositions of high crystallinity proposed thus far include a resin composition containing an aromatic polyester disclosed in JP2005-325231 A and a resin composition formulated from a polyarylene sulfide resin and a liquid crystal polymer such as an aromatic polyester disclosed in JP2005-306955 A. However, liquid crystal polymers such as aromatic polyesters are highly viscous and show poor moldability. Moreover, high viscosity limits the manipulating latitude in combining the polymer with an inorganic filler in the production of a composite material.